Polycarbonates are engineering plastics having good transparency, heat resistance and impact resistance, and are now widely used in the field of electric and electronic appliances, in the field of automobiles, in the field of optical instruments, and in other various industrial fields.
For producing such polycarbonates, known are a method of directly reacting an aromatic dihydroxy compound such as bisphenol A or the like with phosgene (interfacial polycondensation), and a method of transesterifying an aromatic dihydroxy compound such as bisphenol A or the like with a dicarbonate such as diphenyl carbonate or the like in a melt or solid phase (melt polymerization, solid-phase polymerization).
Of the conventional polymerization methods for producing polycarbonates, the interfacial polycondensation method that requires methylene chloride is problematic in that the quality of the products is not good as they often contain chlorine. This is because the products, polycarbonates may contain methylene chloride, but removing methylene chloride from the products is extremely difficult.
On the other hand, the melt polymerization method will be better than the interfacial polycondensation method, as being inexpensive. However, it generally requires long-term reaction at high temperatures falling between 280.degree. C. and 310.degree. C., and its serious problem is that the polycarbonates obtained are inevitably colored. In addition, another problem with the method is that high-molecular polycarbonates could not be obtained.
In order to solve these problems, some proposals were made for transesterification to give polycarbonates. For example, one was to use a specific catalyst (see Japanese Patent Application Laid-Open (JP-A) Hei-6-256479); and another was to add an antioxidant to the reaction system in the latter stage of reaction. Still another was to control and lower the oxygen concentration in the melt transesterification system to a predetermined lower level, thereby improving the hydrolysis resistance and even the color tone of the products, polycarbonates (see JP-A Hei-7-26010). However, all those conventional techniques are still unsatisfactory for solving the problems with polycarbonates which are deteriorated under heat to have poor quality in high-temperature and high-humidity environments, and which are unfavorably colored. In particular, the conventional polycarbonates, if desired to be used in optical materials, could not still have satisfactory properties.